Axis selection in transverse magnetic resonance imaging of the brain: electronic angulation techniques

The cylindrical design of most head coils utilized with current magnetic resonance (MR) imaging units and the necessity of close approximation of the coil to the head to maximize signal-to-noise ratio precludes flexion or extension of the head to any significant degree during of imaging of the brain. For this reason, the canthomeatal line is approximately parallel to the standard transverse magnetic axis. Standard computed tomography (CT) scans in the transverse plane are usually obtained at an approximately 25 degrees angle to Reid's baseline (RBL). This leads to projection differences in viewing and comparing standard transverse MR and CT studies. High convexity lesions which may present anteriorly on a given CT section may present posteriorly on an MR section which appears to be at a comparable level on first inspection. Secondly, one or more transverse MR sections usually display a portion of the occipital lobes behind the cerebellar hemispheres. The region of the tentorium and straight sinus can occasionally give rise to a vermiform appearance (the "AVM artifact"). Thirty patients were studied with MR at a 20-30 degrees angulation to RBL without any loss of image quality and with excellent visualization of the posterior fossa. The effect of transverse axis change on lesion position was demonstrated in five high convexity lesions and by utilizing fixed brain specimens. While clearly MR can accurately localize lesions utilizing orthogonal multiplanar techniques, it is suggested that investigators and clinicians currently performing MR studies of the brain consider potential advantages of electronic angulation techniques for comparative clinical studies and certain research applications.